Reliable and rational techniques for assessing the condition of wire cabling, rods, pipes, and similar elongated ferromagnetic objects such as visual and electromagnetic inspections are known in the art, but depend extensively on the experience and intuition of the human inspector. Serious accidents, various non-scheduled equipment downtimes, and premature replacement as precautionary measures are all consequences of the state of the art, quite apart from the costs involved in the inspection process. Accordingly, it is desirable to provide an improved technique for testing and measuring the actual strength and remaining life in the metallic objects.
One of the primary problems in prior art magnetic inspecting devices is the bulk and weight of most of the devices. Both of these factors limit the applications of the devices and reduce the resolution of the signals that are generated. Test signals are complex and are frequently accompanied by high levels of noise due to non-homogeneities and the coarse construction of objects under test. As a result, data interpretation often is a mixed product of both art and science.
The inspection process addresses two general types of discontinuities that are observed especially in wire cables. The first is a localized discontinuity, such as a broken wire within the cabling, and the second is a distributed discontinuity such as the loss of metallic cross section due to corrosion or abrasion. Both of these discontinuities cause a reduction in metallic cross section and, consequently, affect cable life and strength.
There are several methods of magnetically testing elongated objects such as cables for localized or distributed discontinuities. One of these methods is designated the main flux method and measures the amount of flux that can be carried by the cable between two longitudinally spaced stations. Since the total flux is directly related to the metallic cross sectional area of the object, measurements of the change in flux can be used to detect and measure the loss of area. U.S. Pat. No. 4,096,437 discloses a specific testing device of this type and includes Hall effect devices for measuring the amount of flux in the region of magnetic poles located at the spaced longitudinal stations along a cable. Changes in the cross sectional area caused by corrosion and abrasion can be measured in absolute terms, and relative movement of the cable with respect to the measuring device does not enter into the test parameters. One of the disadvantages, however, is that an extended section of the cable is inspected at any given moment. Therefore, only the average value of the metallic cross sectional area is measured with a considerable loss of resolution. Also, small flaws, such as those caused by broken wires or cluster of wires, and other localized discontinuities are difficult to detect.
Another method of testing employs a saturated magnetic field extending axially through a section of cabling under test and measures changes in leakage flux due to disruptions or breaks in the rope at the surface of the cable. Flux sensors, such as Hall Effect sensors or coils, may measure the changes as sensors and cable are moved relative to one another, and the test signals derived from the sensors may be displayed on a strip-chart recorder that is driven in synchronism with the relative movement of the sensor and cable. U.S. Pat. No. 3,424,976 and U.S. Pat. No. 4,096,437 discloses specific examples of leakage flux detectors.
The advantages of leakage flux systems are that small external and internal flaws, such as broken wires, can be detected and a qualitative indication of corrosion and abrasion is also available. The disadvantages of the prior art systems are that the reduction in cross-sectional area caused by abrasion and corrosion cannot be determined quantitatively, and since the measurements are representative of changes in the leakage flux, signal amplitudes for coils are proportional to the test speeds. As a result of the latter, tachometers are frequently used in connection with an automatic gain control circuit to equalize the signals for recording purposes. This adds complexity and weight to the instruments, and additionally, a certain minimum speed is generally required for a threshold signal. Because of the non-homogeneous structure of wire cabling, test signals are very noisy, and the noise signal cannot be removed by filtering because the differences in the levels of the noise and flaw signals are very small. Still further, because of the requirement for movement between the magnetic device and the cable, the process cannot be carried out at the ends of a cable, which is permanently secured in place.
One known method that is used to overcome the disadvantages listed above has magnets, which induce a saturated magnetic field axially through a short section of the cable as the device and the cable move relative to one another. A sensing coil located in close proximity to the cable between the magnetic field poles detects small changes in leakage flux at the surface of the cable in the saturated condition. The sensed flux changes are applied to an integrator to measure the net variation in flux and correspondingly the total change in cross section. Multiple sense coils mounted on core pieces conforming to the external surface of the cable ensure complete continuity of the inspection process and allow the magnetic device to be installed and removed at intermediate stations along the cable. U.S. Pat. No. 4,096,437 and U.S. Pat. No. 4,659,991 disclose a specific testing device of this type.
The disadvantage of a system using sensing coils is that the coil sensors require fairly elaborate electronic analog circuitry that cannot be accommodated on the sensor head. What is needed is an autonomous sensor head to perform all signal processing aboard the sensor head, or to eliminate supplementary analog circuitry altogether.
Other state of the art devices for nondestructive inspection of elongated magnetically permeable objects such as pipes can be found in U.S. Pat. No. 6,265,870 and U.S. Pat. No. 5,751,144. These devices use an eddy current sensor assembly to inspect elongated permeable objects. The sensor assembly has an auxiliary magnet including first and second auxiliary magnetic poles oppositely polarized relative to each other and spaced from one another for positioning and movement longitudinally relative to an elongated magnetically permeable object to be tested. The auxiliary magnet is interposed between primary magnets of the magnetic inspection device. A ferromagnetic member couples the first and second auxiliary magnetic poles. Compliant pole pieces such as magnetically permanent brushes are coupled to the auxiliary poles and are to be interposed between the auxiliary poles and the object to be inspected. An eddy current sensor is disposed between the auxiliary magnetic poles and includes a sensor body and a means coupled to the ferromagnetic member for urging the sensor body against an opposing surface of the magnetically permeable object to be inspected. The eddy current sensor is able to detect longitudinal discontinuities such as stress corrosion cracks and the like.
The problem with all of the patents listed above is that there is a strong presence of secondary flux between the magnetic poles that can affect the sensitivity of the magnetic flux detector and makes the accurate detection of loss of metallic cross section or localized discontinuities difficult.
Accordingly, it is a general object of the present invention to provide a method and apparatus for quantitatively determining the loss of metallic cross section caused by corrosion, abrasion and other factors, and to also obtain at least a qualitative measurement of localized discontinuities in elongated magnetically permeable objects without the disadvantages mentioned above.